Sizing is a term used in the papermaking art to describe processes which reduce the water absorbency of cellulosic materials such as sheets of paper. This is commonly accomplished by precipitating a nearly fully saponified rosin soap or a free rosin dispersion on the cellulosic fibers with alum to form aluminum resinate. The aluminum resinate coats the exposed surface to give it a hydrophobic character, thereby rendering the surface resistant to wicking by water, inks, and other polar liquids.
Rosin sizes are normally water soluble alkali soaps or a colloidially stabilized dispersions of rosin. A variety of polymeric rosin resin sizes have been developed to address specific coating needs. However, certain rosins are highly prone to crystallization. Crystal formation in such rosins tend to occur during the production of the rosin size, during size storage, or after the size has been diluted to a low solids level.
The traditional process for producing a polymeric resin is well-known (see generally U.S. Pat. Nos. 4,839,413 and 5,216,064, which is hereby incorporated by reference). Commonly a solution polymerization reaction is employed wherein a styrenic monomer and acrylic acid is mixed with a hydrocarbon solvent, a polymerization initiator, and a chain transfer agent. Upon completion of the reaction, the solution is stripped of the solvent to yield the polymer composition.
However, major problems exist with the traditional methods of producing these polymeric resins. For example, these methods require the use of environmentally adverse hydrocarbon solvents. Moreover, as these solvents are not usable or desirable in water-based ink or overprint formulations, the solvents must be stripped from the resulting acrylic polymers (thereby causing a yield loss). This stripping step also adds expense to the process due to both the loss of yield and the energy consumed in performing the stripping. Also, these methods must utilize chain transfer agents to regulate the molecular weight of the resulting resin.
Therefore, an object of this invention is to solve these major problems by disclosing a method of producing rosin-fatty acid vinylic polymer compositions which exhibit properties that make them useful for formulating surface sizing compositions for cellulosic materials.
Another object of this invention is to disclose rosin-fatty acid vinylic polymer surface sizing compositions.